Conventional solid state memories employ micro-electronic circuit elements for each memory bit. Since one or more electronic circuit elements are required for each memory bit (e.g., one to four transistors per bit), these devices can consume considerable chip “real estate” to store a bit of information, which limits the density of a memory chip. The primary memory element in these devices is typically a floating gate field effect transistor device that holds a charge on the gate of field effect transistor to store each memory bit. Typical memory applications include dynamic random access memory (DRAM), synchronous random access memory (SRAM), erasable programmable read only memory (EPROM), and electrically erasable programmable read only memory (EEPROM).
A different type of memory commonly known as a seek-scan probe (SSP) memory uses a non-volatile storage media as the data storage mechanism and offers significant advantages in both cost and performance over conventional memories based on charge storage. Typical SSP memories have storage media made of materials that can be electrically switched between two or more states having different electrical characteristics such as resistance or polarization dipole direction. One type of SSP memory, for example, uses a storage media made of a phase change material that can be electrically switched between a generally amorphous phase and a generally crystalline local order, or between different detectable phases of local order across the entire spectrum between completely amorphous and completely crystalline phases.
SSP memories are written to by passing an electric current through the storage media or applying an electric field to the storage media. Passing a current through the storage media is typically accomplished by passing a current between a sharp probe tip on one side of the storage media and an electrode on the other side of the media. Current SSP memories use probe tip positioned on the free end of a cantilever beam. In an idle state the cantilever beam maintains the probe tip at a certain distance above the storage media, but before the electric field or current can be applied to the storage media the probe tip must usually be brought close to, or in some cases in direct contact with, the storage media.
To bring the probe tip close toward the storage media, the cantilever beam must be deflected such that its free end moves toward the storage media. This is usually accomplished by applying a voltage to the same electrodes that underlie the storage media; when a voltage is applied, the electric field created by the electrode exerts a force on the cantilever beam that deflects the end of the beam toward the electrode. A disadvantage of this approach is that the stiffness of the cantilever beam requires a substantial force for deflection. As a result, a substantial actuation voltage must be applied to the electrodes to actuate the probe. Unfortunately, the voltage that can be applied to the electrodes without damaging them or affecting the storage media is limited, meaning that only a limited deflection of the cantilever is possible or, in cases where the probe tip contacts the storage media, the probe can exert only a limited force on the storage media. In addition, the sense electrode under storage media requires a fixed voltage (usually grounded), which prevents an actuation voltage to be used on the same electrode.